This project will evaluate the potential developmental toxicity of xenobiotics using the whole embryo culture system. This system supports the normal growth and development of rodent embryos in vitro. The adverse developmental effects (e.g. in utero lethality and neural tube defects) of arsenic has been shown in vivo. However, whether these effects are due to the direct effects of the metals or mediated through an effect on maternal homeostasis is unclear. Preliminary studies have addressed the direct effects of As on the conceptus. Mechanistic studies focused on free radical generation. The toxicity of As is dependent upon the concentration and valency state. Antioxidants such as superoxide dismutase, catalase, and glutathione peroxidase were able to partially ameliorate the toxicity of As. We tested the hypothesis that perturbations in glutathione (GSH) were responsible for the effects. There were no significant changes in GSH concentration. However a reduction in GSH peroxidase activity was produced. These results provide important information that oxidative damage is an important part of the pathogenesis. Additionally, antioxidants may be important in preventing malformations in the human population following exposure to environmental oxidants. The mechanisms responsible for cell death will be evaluated in limb bud culture. For these studies, the molecular and cellular effects of antioxidants will be determined. Preliminary evidence suggests that abnormal digit formation occurs as a result of perturbation in reactive oxygen species (ROS). This suggests that the generation of oxygen radicals are important in normal differentiation. We have documented high concentrations of ROS in the interdigital regions and that they are decreased by high concentrations of antioxidants. Changes also occur in the distribution and concentration of Ca+2. It appears to result in a decrease in apoptosis and other differentiation processes.